Power generation systems generate electrical power from various sources including hydropower, wind power, and from the combustion of fuels such as coal, oil and gas. These sources are harnessed to rotate prime movers, typically engines or turbines, that are coupled to power generators, which are in turn coupled to various loads via, for example, a power distribution grid (“grid”).
Such power generation systems employ generators that generally produce electrical power that is proportional in frequency to the rotational speed of a generator, for example, a turbine. Thus, changes in turbine speed may result in changes to the frequency of power generated. Accordingly, the rotational speed of the turbine should be regulated to produce a frequency that matches the requirements of the grid. In situations where the turbine speed has been changed relative to the required grid frequency, or is not sufficient to produce the required frequency, measures must be taken to modulate the generator output frequency to match the grid frequency.
A number of the prior art techniques have been proposed to compensate for changing turbine speeds. These techniques include controlling mechanical variables such as fuel flow rate to regulate turbine rotational speed and using multi-shaft configurations. In addition, various power conversion schemes have been used where power converters are coupled to the output of the generation system.
With present reference to FIG. 1, there is illustrated a block diagram of an exemplary conventional power converter system 100 utilizing a Doubly-Fed Induction Generator (DFIG) converter 102 including a line side converter 104 and a rotor side converter 106 both operating under control of controller 120. Existing DFIG converters generate harmonics from both the line-side converter 104 and rotor-side converter 106. Harmonics generated from the rotor-side converter 106 are feed through DFIG generator 108 and are added together with harmonics of the line-side converter 104, with these combined harmonics feed into the grid 110. Since regulators place limitations on harmonics coupled to the grid, an LRC filter 112 is generally required to be built into the line-side converter 104 to attenuate these harmonics to meet the grid codes. The physical size and cost of producing such filters is significant.
With reference to FIG. 2, there is illustrated a block diagram of an exemplary conventional power converter system 200 utilizing a DFIG converter 202 in association with a three-winding main transformer 216. Inclusion of three-winding main transformer 216 allows operation of the stator of DFIG 208 at a different voltage from the converter 202. As previously noted with respect to FIG. 1, existing DFIG converters generate harmonics from both the rotor and the line side converters. In the instance illustrated in FIG. 2, the three-winding transformer 216 separates the converter line-side filter 212 from the stator by so much impedance that a very large filter 214 must also be added to the stator side. The size and cost of this filter is also significant.
In view of these known issues, it would be advantageous, therefore, to develop systems and methods that would permit reducing component size and related costs for such filters.